North Atlantic wintertime intraseasonal variability and its sensitivity to GCM horizontal resolution

The wintertime extratropical intraseasonal variability in the North Atlantic-European regionhas been analysed, and the ability of an atmospheric general circulation model to simulate itat different horizontal resolutions is discussed. Three runs of the French global spectral model(ARPEGE) are investigated: both medium (T42) and high (T106) resolution, and a conformallystretched T63 version with maximum resolution over the Mediterranean. Circulation features arevalidated against ECMWF analyses. The systematic errors in the simulated variability have beenisolated using space-time spectral and complex principal component analyses. The time-meanstructure of the 500 hPa geopotential height indicates that the model is too zonal, especially athigh resolution, with a stronger than normal synoptic-scale activity. The Atlantic storm trackpenetrates too far eastward over Europe, is too far south and does not veer to the northeast at theexit of the jet. These features may be in part associated with too strong barotropic forcing of themean flow by the synoptic-scale eddies on the equatorward side of the jet. Despite the overly strongzonal mean flow, the high resolution simulation has more realistic storm tracks than the other twoversions. Low-frequency intraseasonal variance is underestimated at each resolution, mainly northof 50° N. Furthermore, the model lacks planetary-scale slow westward travelling waves and doesnot show realistic spatial modes of low-frequency variability at any resolution. Blocking frequencyis underestimated and the blocking patterns are shifted eastward, following the excess of stormtrack penetration over Europe. However, these drawbacks are slightly improved at higher (T106)resolution, especially for blocking.